In general, solvent-based paints will exhibit acceptable adhesion to thermoplastic resin-based compositions regardless of the choice of filler(s) in the composition. However, the use of solvent based paints has been steadily decreasing with the increase in environmentally-conscience suppliers and more stringent regulatory efforts. Accordingly, water-based latex paints have become the standard for a variety of applications.
The ability to use water-based paints to color or cover polymer-based articles is limited at least by interactions between the highly polar aqueous paint solution and the relatively non-polar polymeric material. The carbon-to-carbon linkages that are characteristic of most polymer backbones used for commodity or semi-commodity thermoplastic resins lead to relatively non-polar resins. Two methods for increasing the polarity of polymers include functionalization and compounding or blending.
Functionalization involves incorporation of polar functional groups such as carbonyls, amines, hydroxyls, into the main polymeric chain and/or as side chains. However, the addition of such groups often leads to distinct and detrimental changes in the mechanical properties of the resins. Compounding or blending is an alternative to functionalization of the polymer chain. However, compounding or blending may result in a blended polymer that has extremely different chemistries than any of its components.
Accordingly, manufacturers have long struggled to develop coating compositions capable of coating a variety of substrates that exhibit enhanced paintability (including paintability with water-based paints) and mechanical properties for fabrication (cutting, nailing, routing, etc.), while maintaining acceptable visual appearance, including opacity, gloss, surface appearance, and surface roughness.
For example, one of the most common coatings for MDF interior molding and trim available in the North American market is known as a Gesso coating. Gesso, typically used by suppliers from South America or Asia, is a thick paste that is applied using a wipe-on/wipe-off type process. Drying after coating is required, and a second coating is often applied to provide the surface with a desired look; the second coating must also be dried and buffed. Thus, Gesso coating is relatively labor intensive. Furthermore, although the Gesso coating can yield a smooth, attractive finished surface that is able to hide at least minor imperfections in the surface of the underlying substrate, it can be brittle. Brittleness of the coating may lead to unacceptable handling and fabrication performance, for example, when the molding or trim is sawed, mitered, coped, nailed, and/or routed.
Another common coating available in the North American market is a coating of water-based latex paint. The paint is typically vacuum- or spray-coated onto the substrate. This type of coating is typically used by North American suppliers. Basically, a high volume of water-based latex paint is vacuum-coated or spray-coated to prime the surface of the molding or trim. That coating not only must be dried, as with the Gesso coating, but also must be sanded or buffed. A second coating and drying are also required. Furthermore, unlike the Gesso process, vacuum- and spray-coating can lead to direct telegraphing of the underlying substrate surface to the observable paint surface, revealing structural features of the underlying substrate. Accordingly, the smoothness of the coated surface depends on the quality of the milling of the molding or trim.
Accordingly, there remains a need in the art for coating compositions that, when applied to an underlying substrate, such as, for example, molding or trim, using extrusion technology, can result in a primed substrate having the smooth finish of a Gesso coating but with increased paintability and toughness, and also having acceptable visual appearance.